Prior art concerning solar cells and thin film transistors, TFTs, are known to one knowledgeable in the art. References contained in U.S. Pat. No. 4,128,733, U.S. Pat. No. 6,743,974, U.S. Pat. No. 7,030,313, U.S. 2002/0040727, U.S. 2005/0000566 are cited as prior art and incorporated herein in their entirety by reference.
The present invention addresses the need to increase solar cell efficiency and to further reduce cost over prior art techniques. Typically, present high performance and cost effective solar cell devices are based on bulk silicon materials. Single crystal silicon (sc-Si) cells exhibit single junction (SJ) efficiency approaching ηeff(SJ)˜21%. Unfortunately, prior art techniques for producing sc-Si cells are: (i) costly to manufacture; and (ii) suffer inefficient utilization of the available solar spectrum.
A widely accepted method to reduce the cost of solar cells, by workers in the field, has been to substantially reduce the amount of active material required to form the solar cell via the use of thin film semiconductors. Furthermore, the said thin films are disposed upon low cost substrates such as amorphous glass, polymer and/or metal surfaces.
The simplest and most cost effective method of producing thin films is via use of various deposition methods upon relatively lower cost substrates such as inexpensive glass, polymer and/or metal surfaces or other materials adapted for receiving a layer of silicon.
Typically, the use of low cost substrates places limitation upon thin film semiconductor crystal quality and/or thermal budget required for thin film deposition method of a thin film(s). Low thermal budget deposition of thin films typically results in poor crystal quality semiconductors realized upon amorphous glass substrates. Single semiconductor crystals may nucleate in localized areas upon an initial glass substrate surface, but formation of homogeneous and long range crystal order within the thin film across substantially the entire large area glass substrate is practically impossible without complex post growth recrystallization. Even so, the film quality attained using prior art complex recrystallization techniques is still inferior to bulk single crystal growth techniques, such as, the Czochralski crystal growth (CZ) method.
Single crystal thin film epitaxy is typically done on substrates with intrinsic properties of high single crystal quality, atomically flat surface, and low crystal structure mismatch between the film and substrate. Furthermore, the growing film must adequately wet the substrate surface for layer-by-layer epitaxy, otherwise clustered nucleation growth occurs and thus structurally defective. Glass substrates, by definition, lack all the aforementioned properties except for being able to exhibit extremely flat surfaces via polishing, e.g.; chemical mechanical polishing (CMP) or other techniques such as “float glass”. Uniform and flat thin film semiconductors are typically best deposited onto glass substrate surfaces in amorphous form. Typically, flat and uniform amorphous semiconductors can be deposited onto glass substrates at room temperature or modest substrate temperatures. Subsequent thermal processing of an amorphous Semiconductor-on-Glass (a-SoG) article is then required to transform the amorphous thin film semiconductor into the desired polycrystalline form. For example, Nickel induced crystallization allows amorphous Si to be recrystallized via catalytic action into poly-Si. More desirable of the polycrystalline forms are thin film semiconductors exhibiting large domains (grain size˜0.1-10 microns) in lateral and/or vertical dimensions relative to the film growth direction. Thin films exhibiting larger lateral grain dimension than film thickness enable advantageous transport of electronic carriers parallel to the film/substrate surface. Direct deposition of thin film semiconductors onto glass substrates without complex post processing results in polycrystalline (pc), microcrystalline (mc), nanocrystalline (nc) and/or amorphous (a) semiconductor thin films.
It is well known that large area single crystal thin film semiconductors cannot be directly deposited epitaxially upon glass and/or amorphous substrates. High quality single crystal thin film semiconductors on large area glass substrates are commercially feasible at present using only thin film layer transfer techniques, such as wafer bonding and etch back technique. That is a thin film layer transfer technique comprises the steps of thinning and/or separation of a single crystal thin film from a single crystal bulk semiconductor to the film thickness required and subsequent transfer of said film onto a surface of an acceptable substrate. The single crystal thin film is then bonded to the substrate forming a thin film semiconductor-on-substrate article; in some embodiments an acceptable substrate is glass; alternatively other types of substrates are acceptable.
Layer-transfer methods provide a practical means of cost effective manufacture of single crystal thin film semiconductor disposed upon inexpensive substrates. The design compromise for the structural quality of the thin film is therefore between cost and whether single-crystal or polycrystal thin films are required for a specific application.
Prior art techniques to date fail to address the implication of the intrinsic chemical property of cheap substrate composition; for instance, glass is typically alkali-silicate-based and quite severely impacts the electronic and/or optoelectronic performance of a thin film semiconductor.
It is therefore not well known by researchers in the field of thin film semiconductor manufacture, the fact that it is not a simple matter of replacing the substrate with a cheap alternative, such SiO2-based glass, because of the typically chemically disruptive influence on at least one of the thin film electronic and/or optical and/or chemical and/or mechanical properties. For an example case of using cheap SiO2-based glass substrate to form SoG article, regardless of the thin film semiconductor manufacture technique (i.e.; via layer-transfer or direct deposition), the chemical composition of cheap glass poses severe contamination concerns for electronic and optoelectronic performance of thin film devices.